Too much of anything is bad, as I am observing more and more.
Whenever someone is being overly optimistic, I tend to counterbalance that with outlining the risks or challenges. Not to encourage them not to try, just to be prepared for the bumps in the road on the way to achieving it. Not getting bucked off.
Same with negativity. I know the negative narrative that runs in the back of many or our minds and how of us simply harness the power of that and figure out how to leverage it for success.
In each of those extremes, too much of anything bad or otherwise is not good. Except maybe Vitamin C. ;)